Wide band balun

ABSTRACT

A wide band balun for connecting an unbalanced coaxial input line to a balanced load. The input line is separated into a first and a second portion by a gap in its outer conductor. The balun achieves wide band operation by the use of two short circuited stubs in shunt with respect to the gap and an open circuited stub in series with respect to the gap.

United States Patent Woodward [451 Apr. 11, 1972 [54] WIDE BAND BALUN2,606,964 8/1952 Gluyas, Jr. .ssslge 72 Inventor: Oakle McDonald Woodard P c t i NJ y w m e on Primary Examiner-Paul L. GenslerAttorneyEdward J. Norton [73] Assrgnee: RCA Corporation [22] Filed: June25, 1970. [57] ABSTRACT [21] Appl. No.: 49,745' A wide band balun forconnecting an unbalanced coaxial input line to a balanced load. Theinput line is separated into a first and a second portion by a gap inits outer conductor. The balun achieves wide band operation by the useof two short 58 Field of Search ..333/26, 25; 343/859 circuited stubs inshum with respect to the p and an p circuited stub in series withrespect to the gap. [56] v References Clted 8 Claims, 5 Drawing FiguresUN lTED STATES PATENTS v 3,497,832 2/1970 Cohn 333 2934 PATENTEUAP R 1 1I972 SHEET 1 UF 2 l N VEN TOR. Oakley McDonald Woodward BY WM W;

ATTORNEY PATENTEDAPR 11 m2 sum 2 UF 2 Fig. 3.

Fig. 4.

Oakley McDOMIc I Qi ZwS md B Y M yhQ ATTORNEY WIDE BAND BALUN Theinvention herein described was made in the course of or under a-contractor subcontract thereunder with the Department of the Air Force.

This invention relates to baluns for balanced coaxial lineto a balancedload.

There are many devices known in the prior art for performing the balunfunction. A typical prior art balun is shown in U.S. Pat. No. 2,925,566,which shows (i) the typical interconnections between an unbalanced inputcoaxial line and two output coaxial lines connected to a balanced loadand (ii) the use of a high impedance shunt stub in order to maintain alow voltage standing wave ratio over a large bandwidth. I An object ofthe present invention is to provide a balun having a' broader standingwave ratio bandwidth than previously realiable.

Inthe present invention, an unbalanced coaxial input line connecting anuntor 22 and an'inne'r conductor 23. The outer conductor 20 of line 18is electrically connected to the outer conductor ill of having a firstand a second portion is provided. The first portion of the line isseparated from the second portion of the line by a gap in the outerconductor of the input line. The second portion of the input line isterminated in an open circuitand provides a series stub with respect tothe gap. First and second coaxial output lines are provided. The outerconductor of the first output line is connected to the outer conductorof the first portion of the input line. The outer conductor of thesecond output line is connected to the outer conductor of the secondportion of the input line. Means are provided for electricallyconnecting the inner conductor of the first output line to the outerconductor of the second output line and means are provided forelectrically connecting the outer conductor of the first portion of theinput line to the inner'conductor of the second output line. Additionalmeans provide short circuit between a first region on the outerconductor of the first portion of the input line and a second region onthe outer conductor .of the second portion of the input line. The firstand second regions are substantially a quarter wavelength from the gapat a predetermined frequency.

IN THE FIGURES FIG. 1 illustrates a balun invention;

FIG. 2 is a cross-sectional view of the-embodiment shown in FIG. 1;

FIG. 3 is an equivalent circuit representation of the balun shown inFIG. 1;

FIG. 4 shows a representation of the balun of FIG. 1 in conjunction withan impedance transformer; and

FIG. 5 is a sketch of curves showing standing wave ratio versusfrequency for various balun arrangementsl The balun shown in FIG. 1 hasa coaxial input line having an outer conductor 11 and an inner conductor12. The unbalanced input line 10 has a gap 13 in its outer conductor 11separating a first portion of input line shown generally as 14 and asecond portion of input line shown generally as 15. The gap 13 istypically very small and may be on the order of less than 0.005wavelengths at the center frequency of operation. The gap 13 should besmall enough to avoid inductive loading from portion 14 to portion 15 ofthe line, yet large enough to avoid voltage breakdowns in the region ofthe separation. The second portion of the input line 15 has an opencircuit termination 16. The second portion of input line 15 has an innerconductor 17 which is larger than the inner conductor '12 of the firstportion of the input line. The second portion of input line 15 providesa series stub of relatively low characteristic impedance with respect tothe first portion of input line 14.

The input line 10 which may typically have a characteristic impedance of37.5 ohms is connected to a balanced load (not shown), which maytypically be 150 ohms. The 150 ohm load is divided into two equal 75 ohmloads and the two 75 ohm loads are connected to two equal length 75 ohmcoaxial lines 18 and l9which branch and enter the balun from oppositeends. Output line 18 has an outer conductor and an inner conductor 21.The second output line 19 has an outer conducpreferred embodiment of thepresent the first portion of input line 14. The outer conductor 22 ofline 19 is electrically connected to the outer conductor 11 alongthesecond portion of the input line 15.

In the area of the gap 13, the outer conductor 11 of the first portionof the input line 14 is connected to the inner conductor 23 of line 19by line 24. Also, in the area of. the gap 13', inner conductor 21 ofline 18 is connected to the outer conductor 22 of line 19 by line 25.The interconnection of lines 18 and 19 by conductors 24 and 25 place thetwo 75 ohm loads in parallel across the gap 13.

It is seen from the geometric symmetry that the balun sees identicalloads in each of the output lines 18 and 19 when viewed from the gap 13.Therefore the balun is inherently balanced at all frequencies. Thevoltage appearing between conductors l1 and 12 in the vicinity of thegap 13 is supplied from a generator (not shown) connected to the inputline 10. The voltage in the vicinity of the gap 13 drives currentsthrough the two 75 ohm loads that are equal in magnitude and opposite indirection.

A cylindrical conducting member 26 having ends 27 and 28 is positionedabout the input line 10 and output lines 18 and 19 such that the outerconductor 1 1 of the first portion of line 14 is short-circuited to theouter conductor 11 of the second portion of line 15.'The short circuitis provided by end 27, at a wavelength from gap 13 along portion 15 atthe center frequency of operation. Member 26 provides two shortcircuited shunt stubs, of relatively high impedance with respect to theinput line 10, which are in series with each other. This resultingreactance is in parallel with'the combination of the two 75 ohm loads.

In FIG. 2, it is shown that the balun may be thought of as a highcharacteristic impedance coaxial line, one-half wavelength long at thecenter frequency of operation, with short circuits 28 and 27 at eachend. The coaxial line comprises an outer conductor formed by member 26and dual inner conductors 11 and 22 placed side by side.

The equivalent impedance transformation circuit of the balun of FIG. 1is shown in FIG. 3 where the input line 10 is represented by terminalsA, B, the series stub is represented by C and L and where the shuntstubs are represented by C L and C314 respectively. The two 75 ohm loadsare in parallel across the shunt stubs as shown.

Improved bandwidth is achieved since the shunt stubs C L and C 1 eachhaving a relatively high characteristic impedance, are in series withrespect to each other and are con nected in parallel across the. twoloads. The reactancefrequency slope of the series stub C,L is chosen soas to reduce the input reactance seen at terminals AB at or near thehigh and low. frequency limits of the band, thus, giving a low standingwave ratio over the entire frequency band.

At the center frequency of operation, F the series stub C 1. is designedto have zero reactance and the shunt stubs are designed to be resonant.Under these conditions, the impedance looking into terminals AB is apure resistance of 37.5 ohms, giving a voltage standing wave ratio(VSWR) of 1.0 measured on a 37.5 ohm line. The VSWR increases as thefrequency-shifts-toward the band limits. Some VSWR improvement can beobtained by renormalizing the input impedance to a slightly lower valuethan 37.5 ohms. The net effect of renormalizing the input impedance isthat the balun will not be matched at F but the VSWR is reduced at theband limits.

FIG. 4 shows the means by which renormalizing of the input impedance maybe accomplished. Input line 10 is connected to a wide band impedancetransformer 30 which may be a conventional type of Tchebyschefi' taperedtransformer made of five cascaded quarter wave steps. Transformer 30 isthen The curves of FIG. show the effects upon the VSWR for various balunconfigurations. Curve 40 is a typical plot for a balun of the type knownin the prior art. That is, it is a curve of the VSWR for a balun withouta series stub and without any renormalization. Curve 40 shows the balunto be matched at the center frequency P with the standing wave ratioincreasing rapidly on either side of F Curve 42 shows the effect ofadding a series stub to the balun. Again, the balun is matched at thecenter frequency of operation, F however, the addition of the seriesstub causes a slower change in standing wave ratio as the frequency ischanged from F Curve 44 shows the effect on the VSWR characteristic of abalun using a series stub and an impedance transformer for the purposesof renormalization. The balun is now matched at two frequencies F and Fbut is not matched at the center frequency of operation F The effect ofthe series stub and the impedance transformer is to substantially widenthe usable bandwidth in terms of the input VSWR of the balun.

Bandwidths of over 6.6:1 have been achieved in the high frequency (HF)range for a standing wave ratio of 1.08:1 utilizing the presentinvention in a balun wherein the shunt stubs had a characteristicimpedance of l ohms and the series stub had a characteristic impedanceof 6 ohms.

By employing sufficiently large size coaxial line elements and byinstalling small capacity plates with corona rings at the coaxial lineends in the region of the gap, the balun may be designed to handle veryhigh peak powers.

The present invention may be practiced in the preferred embodiment shownin FIG. 1 or other embodiments without departing from the spirit orscope of the present invention. For example, another embodiment of thebalun is to utilize two coaxial lines above a ground plane wherein oneline would be comparable to the input line 10 of FIG. 1, and the secondline would be comparable to output lines 18 and 19. Two short-circuitingmembers would then be arranged to join the ground plane to the outerconductor of the input line with each shortcircuiting member being aquarter wavelelength away from the gap region.

I claim:

1. A wide band balun comprising:

an unbalanced coaxial input transmission line, said input line having afirst and a second portion, said first portion being separated from saidsecond portion by a gap in the outer conductor of said input line, saidsecond portion being terminated in an open circuit for providing aseries stub with respect to said gap;

a first coaxial output line, the outer conductor of said first outputline being electrically connected to the outer conductor of said firstportion of said input line;

a second coaxial output line, the outer conductor of said second outputline being electrically connected to the outer conductor of said secondportion of said input line;

means located only in the region of said gap for electrically connectingthe inner conductor of said first output line to the outer conductor ofsaid second output line;

means located only in the region of said gap for electrically connectingthe outer conductor of said first portion of said input line to theinner conductor of said second output line; and

means for providing a short circuit between a first region on the outerconductor of said first portion of said input line and a second regionon the outer conductor of said second portion of said input line, saidfirst and second regions each being substantially a quarter wavelengthfrom said gap at a predetermined frequency.

2. The balun according to claim 1, further comprising:

impedance transformation means interposed between an unbalanced coaxialline and said first portion of said input line for transforming theimpedance of said first portion of said input line to a desiredimpedance.

3. The balun according to claim 1, wherein the outer conductor of saidfirst output line is electrically connected to and coextensive with saidfirst portion of said input line for a length of at least a quarterwavelength from said gap at said predetermined frequency and wherein theouter conductor of said second output line is electrically connected toand coextensive with said second portion of said input line for a lengthof at least a quarter wavelength from said gap at said predeterminedfrequency.

4. The balun according to claim 1 wherein the impedance of said seriesstub is substantially lower than the impedance of said first portion ofsaid input line. i

5. The balun according to claim 1 wherein the means f0 providing a shortcircuit comprises a conductive cylindricallike member, the base at oneend of said member being connected to the outer conductor of said firstportion of input line in said first region, the base at the other end ofsaid member being connected to the outer conductor of said secondportion of input line in said second region.

6. A wideband balun, comprising:

an unbalanced coaxial input line, said input line having a first portionexhibiting a first characteristic impedance and a second portionexhibiting a second characteristic impedance, said second characteristicimpedance being lower than said first characteristic impedance, saidinput line further having a gap in the outer conductor thereofseparating said first and second portions of said input line, saidsecond portion of said input line being terminated in an open circuit:

a first coaxial output line, said first coaxial output line having anouter conductor which is coextensive with and electrically connected tothe outer conductor of the first portion of said input line for apredetermined length along said first portion of said input line;

a second coaxial output line, said second coaxial output line having anouter conductor which is coextensive with and electrically connected tothe outer conductor of said second portion of said input line for apredetermined length along said second portion of said input line;

means for connecting the inner conductor of said first coaxial outputline to the outer conductor of said second coaxial output line only inthe region of said gap;

means for connecting the outer conductor of said first portion of saidinput line to the inner conductor of said second coaxial output lineonly in the region of said gap; and

means for providing a short circuit between a first region on the outerconductor of said first portion of said input line and a second regionon the outer conductor of said second portion of said input line, saidfirst and second regions each being substantially a quarter wavelengthfrom said gap at a predetermined frequency of operation.

7. The balun according to claim 6, including impedance transformationmeans coupled to said first portion of said input line for transformingsaid first characteristic impedance to a desired impedance.

8. A wide band balun for connecting an unbalanced coaxial line to abalanced line, comprising:

an unbalanced coaxial input transmission line, said input line having afirst portion at a first characteristic impedance and a second portionat a second characteristic impedance, said second characteristicimpedance being lower than said first characteristic impedance, saidinput line further having a gap in the outer conductor thereofseparating said first and said second portions of said input line, saidsecond portion of said input line having an open circuit termination,said second portion of said input line providing a low impedance seriesstub with respect to said S p impedance transformation means coupled tosaid first portion of said input line for matching said firstcharacteristic impedance to a desired impedance;

a first coaxial output transmission line, said first coaxial output linehaving an outer conductor which is coextensive with and electricallyconnected to the outer conductor of a second coaxialoutput transmissionline, said second coaxial output line having an outer conductor which iscoextensive with and electrically connected to the outer conductor ofsaid second portion of said input line for a predetermined length alongsaid second portion of said input line line;

means for connecting the inner conductor of said first output line tothe outer conductor of said second output line only in the vicinity ofsaid gap;

means for connecting the outer conductor of said first portion of saidinput line to the inner conductor of said second output line only in thevicinity of said gap; and

a conductive cylinder surrounding said coextensive length of said firstportion of input line and said first output line and said coextensivelength of said second portion of input line and said second output line,said cylinder being electrically connected at one end thereof to theouter conductor of said first portion of input line at a quarterwavelength from said gap at a predetermined frequency and electricallyconnected at the other end thereof to the outer conductor of said secondportion of input line at a quarter wavelength from said gap at saidpredetermined frequency, said cylinder providing two shunt stubs withrespect to said gap; said first output line inner conductor and saidsecond output line inner conductor comprising said balanced line.

' I i 2 1 i

1. A wide band balun comprising: an unbalanced coaxial inputtransmission line, said input line having a first and a second portion,said first portion being separated from said second portion by a gap inthe outer conductor of said input line, said second portion beingterminated in an open circuit for providing a series stub with respectto said gap; a first coaxial output line, the outer conductor of saidfirst output line being electrically connected to the outer conductor ofsaid first portion of sAid input line; a second coaxial output line, theouter conductor of said second output line being electrically connectedto the outer conductor of said second portion of said input line; meanslocated only in the region of said gap for electrically connecting theinner conductor of said first output line to the outer conductor of saidsecond output line; means located only in the region of said gap forelectrically connecting the outer conductor of said first portion ofsaid input line to the inner conductor of said second output line; andmeans for providing a short circuit between a first region on the outerconductor of said first portion of said input line and a second regionon the outer conductor of said second portion of said input line, saidfirst and second regions each being substantially a quarter wavelengthfrom said gap at a predetermined frequency.
 2. The balun according toclaim 1, further comprising: impedance transformation means interposedbetween an unbalanced coaxial line and said first portion of said inputline for transforming the impedance of said first portion of said inputline to a desired impedance.
 3. The balun according to claim 1, whereinthe outer conductor of said first output line is electrically connectedto and coextensive with said first portion of said input line for alength of at least a quarter wavelength from said gap at saidpredetermined frequency and wherein the outer conductor of said secondoutput line is electrically connected to and coextensive with saidsecond portion of said input line for a length of at least a quarterwavelength from said gap at said predetermined frequency.
 4. The balunaccording to claim 1 wherein the impedance of said series stub issubstantially lower than the impedance of said first portion of saidinput line.
 5. The balun according to claim 1 wherein the means forproviding a short circuit comprises a conductive cylindrical-likemember, the base at one end of said member being connected to the outerconductor of said first portion of input line in said first region, thebase at the other end of said member being connected to the outerconductor of said second portion of input line in said second region. 6.A wideband balun, comprising: an unbalanced coaxial input line, saidinput line having a first portion exhibiting a first characteristicimpedance and a second portion exhibiting a second characteristicimpedance, said second characteristic impedance being lower than saidfirst characteristic impedance, said input line further having a gap inthe outer conductor thereof separating said first and second portions ofsaid input line, said second portion of said input line being terminatedin an open circuit: a first coaxial output line, said first coaxialoutput line having an outer conductor which is coextensive with andelectrically connected to the outer conductor of the first portion ofsaid input line for a predetermined length along said first portion ofsaid input line; a second coaxial output line, said second coaxialoutput line having an outer conductor which is coextensive with andelectrically connected to the outer conductor of said second portion ofsaid input line for a predetermined length along said second portion ofsaid input line; means for connecting the inner conductor of said firstcoaxial output line to the outer conductor of said second coaxial outputline only in the region of said gap; means for connecting the outerconductor of said first portion of said input line to the innerconductor of said second coaxial output line only in the region of saidgap; and means for providing a short circuit between a first region onthe outer conductor of said first portion of said input line and asecond region on the outer conductor of said second portion of saidinput line, said first and second regions each being substantially aquarter wavelength from said gap at a predetermined frequency ofoperation.
 7. The balun according to claim 6, including impedancetransformation means coupled to said first portion of said input linefor transforming said first characteristic impedance to a desiredimpedance.
 8. A wide band balun for connecting an unbalanced coaxialline to a balanced line, comprising: an unbalanced coaxial inputtransmission line, said input line having a first portion at a firstcharacteristic impedance and a second portion at a second characteristicimpedance, said second characteristic impedance being lower than saidfirst characteristic impedance, said input line further having a gap inthe outer conductor thereof separating said first and said secondportions of said input line, said second portion of said input linehaving an open circuit termination, said second portion of said inputline providing a low impedance series stub with respect to said gap;impedance transformation means coupled to said first portion of saidinput line for matching said first characteristic impedance to a desiredimpedance; a first coaxial output transmission line, said first coaxialoutput line having an outer conductor which is coextensive with andelectrically connected to the outer conductor of the first portion ofsaid input line for a predetermined length along said first portion ofsaid input line; a second coaxial output transmission line, said secondcoaxial output line having an outer conductor which is coextensive withand electrically connected to the outer conductor of said second portionof said input line for a predetermined length along said second portionof said input line line; means for connecting the inner conductor ofsaid first output line to the outer conductor of said second output lineonly in the vicinity of said gap; means for connecting the outerconductor of said first portion of said input line to the innerconductor of said second output line only in the vicinity of said gap;and a conductive cylinder surrounding said coextensive length of saidfirst portion of input line and said first output line and saidcoextensive length of said second portion of input line and said secondoutput line, said cylinder being electrically connected at one endthereof to the outer conductor of said first portion of input line at aquarter wavelength from said gap at a predetermined frequency andelectrically connected at the other end thereof to the outer conductorof said second portion of input line at a quarter wavelength from saidgap at said predetermined frequency, said cylinder providing two shuntstubs with respect to said gap; said first output line inner conductorand said second output line inner conductor comprising said balancedline.